Lecture Notes: Separations and Purifications Practice Problems

Introduction

• Presenter: Iman (YouTube Channel)
• Focus: Practice problems related to separations and purifications lecture.

Problem 1: Fractional Distillation

• Concept: Effective for separating two liquids that boil within a few degrees of each other.
• Answer Choices:
  • A: 40° and 10° C (30° difference)
  • B: 77° and 80° C (3° difference) ✅ (Correct Answer)
  • C: 184° and 205° C (21° difference)
  • D: 184° and 100° C (84° difference)

Problem 2: Extracting Benzoic Acid

• Concept: Sodium hydroxide converts benzoic acid to sodium benzoate, which dissolves in aqueous solution.
• Answer Choices:
  • A: Tetrahydrofuran (incorrect)
  • B: Aqueous HCl (incorrect)
  • C: Aqueous Sodium Hydroxide ✅ (Correct Answer)
  • D: Water (incorrect)

Problems 3 & 4: Thin-Layer Chromatography (TLC)

• Problem 3 RF Values:
  • Concept: Hexane is less polar than ether, decreasing the RF value of polar compounds.
  • Answer: D ✅ (Decreased RF value)
• Problem 4 Column Chromatography Elution:
  • Concept: Less polar compounds travel faster, elute first.
  • Answer: Napthalene ✅ (Correct Answer is D)

Problem 5: Extraction Procedure

• Concept: Multiple extractions with smaller amounts are more effective.
• Answer: D ✅ (Four successive extractions)

Problem 6: Chromatography Solvent Systems

• Concept: In reverse-phase chromatography, the most polar compound travels farthest.
• Answer: A ✅ (Compound 3 is the most polar)

Problem 7: Extraction with Methylene Chloride

• Concept: Denser methylene chloride forms the bottom organic layer.
• Answer: B ✅ (Bottom layer)

Problem 8: Properties of Silica Gel in TLC

• Concept: Silica gel is polar, used to selectively attract polar solutes.
• Answer: B ✅ (Polarity)

Problem 9: Separation Techniques

• Concept: Combination of filtration, extraction, and evaporation.
• Answer: B ✅ (Filtration, Extraction, Evaporation)

Problem 10: Simple Distillation

• Concept: Effective for compounds under 150° C and at least 25° difference in boiling points.
• Answer: C ✅ (Correct Answer)

Problem 11: Chromatography Technique for Biological Effectors

• Concept: Affinity chromatography using target proteins or antibodies.
• Answer: C ✅ (Affinity Chromatography)

Problem 12: Chromatography for Insulin and Tryptophan Separation

• Concept: Size exclusion chromatography for molecules of different sizes.
• Answer: Size Exclusion Chromatography

Problem 13: Definitions of Mobile Phase

• Concept: Gas effluent (Gas chromatography) and liquid (Paper chromatography) are mobile phases.
• Answer: B ✅ (Mobile Phase)

Problem 14: Gravity Filtration Error

• Concept: Cold solvent causes product to precipitate out, found in residue.
• Answer: C ✅ (Product in residue)

Problem 15: Example of Ion Exchange Chromatography

• Concept: Charged proteins stick to charged resin column; rest washes through.
• Answer: B ✅ (Ion Exchange Chromatography)

Conclusion

• Practice problem set finishes lecture series on separations and purifications for MCAT organic chemistry.
• Encouragement and wishes for success on exams.
• Invite for questions and comments.